This invention relates to a catalytic structure and a process for producing such structure. More particularly, it relates to the production of a catalytically active structure which is especially suited for high temperature use in a gaseous stream. In a specific embodiment, the present invention is directed to the preparation of catalysts comprised of an alloy consisting essentially of Cr-Ni-Cu in a configuration which is highly effective for the purification of automobile exhaust and industrial waste gases.
It is well known that catalysts may be used in a variety of forms in a catalytic bed, and that the catalyst structural design plays a marked role in determining catalyst suitability for a particular process. The inter-relationships of the catalyst composition, design and process conditions are highly complex and often difficult to isolate from each other. In many processes using fixed bed catalysts, where large volumes of gaseous or liquid streams are passed through the reactor, it is advantageous to have a high surface area of the catalyst exposed relative to the amount of catalyst so as to facilitate contact of the reactants at the surface. The availability of the catalyst surface is particularly important in certain gaseous reactions, such as the purification of automobile engine exhaust streams and nitric acid plant tail gas streams, where the gas may pass through or adjacent to the catalyst at a very high linear velocity. In such processes, conventional particulate catalysts, e.g., pellets, chips, cylinders, spheres, and the like, have the disadvantage that they contribute to back-pressure build-up in the system. Also, the particulate catalysts are more subject to attrition than a catalyst in which the movement is more restricted. In such types of reactions a structural form which will permit free passage of the reacting gases relative to the surface exposed will greatly enhance the suitability of an active catalytic material. Typical of the high surface area -- larger open area stationary catalyst structures are metal gauze, woven metal screens, expanded metal mesh, honeycomb, metal foam, and knitted metal mesh. These may be massive catalyst structures or the catalyst may be deposited on a support of suitable design. The supports which may be, for example, of metal, ceramic, or glass, may participate in the catalytic activity or may serve merely to present the catalyst in a suitable structural pattern
U.S. Pat. No. 3,928,235 filed Nov. 12, 1973 describes catalysts which are useful for high temperature applications, particularly for the purification of automotive exhaust streams. These catalysts are comprised of alloys containing at least chromium and copper. The chromium-copper alloys are effective oxidation catalysts, e.g., for oxidizing CO to CO.sub.2, and alloys of the chromium-nickel-copper are particularly suitable for the removal of oxides of nitrogen as well as carbon monoxide and hydrocarbons from such streams. While the catalysts disclosed exhibit excellent activity and selectivity for many reactions, they are expensive to fabricate into complex shapes by conventional hot and cold working techniques because of limited malleability at room temperature.
It is an object of the present invention to provide a catalyst in a form which is especially suited for reactions involving high velocity gas or liquid flow reactants.
Another object is to provide a catalyst of a structural design which has a high surface area and large open area relative to the amount of catalyst.
A further object is to provide a catalyst structure consisting of a coating of the catalytically active material on a pre-formed structure which is stable at high temperatures.
A still further object is to provide a catalyst structure comprising an alloy consisting essentially of chromium, nickel and copper, the catalyst being developed on a pre-formed metal mesh substrate, which is made of a nickel and chromium-containing alloy.
The objects and advantages will become apparent from the following description.
In accordance with U.S. Pat. No. 3,925,259 a coherent catalyst comprising an alloy of chromium and copper is provided in a suitable structural form for permitting high rates of mass transfer and low resistance to flow by means of liquid-phase sintering of the alloy prepared as a pre-alloyed powder. In a preferred embodiment, the catalyst or precursor alloy thereof is provided as a coherent adherent coating of a nickel, chromium and copper on a substrate of suitable design. The present application concerns a further method of fabricating supported catalytic structures.